45PSS, I thought marks on the back of the lens were not such an issue. Or have I got that the wrong way round, and it's the front that needn't be in perfect condition?

Scratches on/in the rear element are more detrimental to image quality than scratches on the front.
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The auction lens has what looks like mold spots inside the lens. It could also be coating separation or fungus that etches the glass.
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That is a "Full Sync Shutter". The Black M is used with the 100 and 200 speeds, the Red M is used with speeds 50 and longer. F-X is used for gas filled flash bulbs or electronic flash at any of the available speeds. Off turns the flash sync off so no flash occurs with flash equipment connected._________________The best camera ever made is the one that YOU enjoy using and produces the image quality that satifies YOU.